Cellular Growth Factors Generally
In recent years, significant attention has been paid to the large number of mitogenic factors, more commonly referred to generically as cellular growth factors, which have been found to be produced by various types of human cells. The outstanding characteristic common to these growth factors is their ability to stimulate proliferation in vitro of a number of cell types. In their normal state, diploid cells in culture rely on the presence of exogenous growth factors to initiate division, and in the absence of these factors, cells will leave the cell cycle and remain in G.sub.1 /G.sub.0 until stimulated externally. On the other hand, transformed cells (e.g., cancerous cells) do not rely upon exogenous growth factors to stimulate mitosis, but rather appear to have some internal control mechanism which permits continuous cell division without any external stimulus. It is in this regard that the so-called "growth factors" have been actively studied. A significant body of evidence suggests that these mitogens may play a role as "transforming proteins" which in vivo are associated with causing the loss of control over growth exhibited by tumor cells. In fact, a number of observations indicate that certain growth factors are routinely made and released by certain human tumor cells in vitro.